This invention relates to the display of digital data. More specifically, this invention relates to apparatus for increasing the dynamic range in a cathode ray tube display of X-ray tomographic images.
Photographic displays of X-ray absorption characteristics have, for over half a century, been a principal tool for medical diagnosis. Variations in the density of a silver image on a photographic transparency are utilized to represent the X-ray absorption characteristics of internal body tissues. Conventional X-ray photography is, however, limited to the display of superimposed shadows of bodily organs lying in a transmission path. The characteristics of silver halide photographic emulsions tend to limit the X-ray absorption density resolution of images so displayed.
More recently, a method of X-ray tomography has been utilized to provide high resolution, sectional displays of internal tissue structures. In accordance with this method, X-ray transmission characteristics are measured along a plurality of paths through an object undergoing examination. Images of X-ray absorption densities within the object are then constructed by computations on the X-ray transmission data. The calculations are most advantageously performed in a digital computer and the images may be displayed, for example, on a cathode ray tube. A specific method of X-ray tomographic imaging is, for example, described in U.S. Pat. No. 3,778,614 to Hounsfield, which is incorporated by reference, as background material in this disclosure.
X-ray tomographic methods are capable of producing images having far greater absorption density resolution than images produced by photographic techniques. For example, present tomographic image reconstruction methods are capable of quantitizing X-ray absorbtion density measurements into 256 or more separate levels. Cathode ray tube data displays are incapable, however, of displaying more than approximately 15 distinct gray-scale levels.
Significant medical information, for example, the presence of tumors in soft tissue, is often represented by minimal level changes in tomographic image density. The detection of such level changes is often accomplished by a process wherein a radiologist views a computer-generated image having a limited gray-scale range and, on the basis of his observation and experience, interactively modifies the computer program to present significant density information within the dynamic range of the image display. The observations and procedures inherent in this process are, of course, highly subjective so that a lengthy series of iterations may be required to obtain an optimum display. Each modification to the computer program will, in general, cause an interruption in the processing of other tomographic image information and, by distracting the radiologist's attention from the display screen will lengthen the image interpretation process.
It is often useful to determine the absolute X-ray absorption density displayed in a tomographic image. If the output signal of a digital computer has been modified to optimize a cathode ray tube display, such absolute density measurements may require further calculations to recover the absolute density information.